From f80d034086d5bfcfd3bf4ab6f52b2df78c3ad2fa Mon Sep 17 00:00:00 2001 From: Mark Rutland Date: Fri, 13 Nov 2020 12:49:21 +0000 Subject: arm64: ensure ERET from kthread is illegal For consistency, all tasks have a pt_regs reserved at the highest portion of their task stack. Among other things, this ensures that a task's SP is always pointing within its stack rather than pointing immediately past the end. While it is never legitimate to ERET from a kthread, we take pains to initialize pt_regs for kthreads as if this were legitimate. As this is never legitimate, the effects of an erroneous return are rarely tested. Let's simplify things by initializing a kthread's pt_regs such that an ERET is caught as an illegal exception return, and removing the explicit initialization of other exception context. Note that as spectre_v4_enable_task_mitigation() only manipulates the PSTATE within the unused regs this is safe to remove. As user tasks will have their exception context initialized via start_thread() or start_compat_thread(), this should only impact cases where something has gone very wrong and we'd like that to be clearly indicated. Signed-off-by: Mark Rutland Cc: Christoph Hellwig Cc: James Morse Cc: Will Deacon Link: https://lore.kernel.org/r/20201113124937.20574-2-mark.rutland@arm.com Signed-off-by: Catalin Marinas --- arch/arm64/kernel/process.c | 17 ++++++++--------- 1 file changed, 8 insertions(+), 9 deletions(-) (limited to 'arch/arm64/kernel') diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c index 4784011cecac..855137daafbf 100644 --- a/arch/arm64/kernel/process.c +++ b/arch/arm64/kernel/process.c @@ -422,16 +422,15 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start, if (clone_flags & CLONE_SETTLS) p->thread.uw.tp_value = tls; } else { + /* + * A kthread has no context to ERET to, so ensure any buggy + * ERET is treated as an illegal exception return. + * + * When a user task is created from a kthread, childregs will + * be initialized by start_thread() or start_compat_thread(). + */ memset(childregs, 0, sizeof(struct pt_regs)); - childregs->pstate = PSR_MODE_EL1h; - if (IS_ENABLED(CONFIG_ARM64_UAO) && - cpus_have_const_cap(ARM64_HAS_UAO)) - childregs->pstate |= PSR_UAO_BIT; - - spectre_v4_enable_task_mitigation(p); - - if (system_uses_irq_prio_masking()) - childregs->pmr_save = GIC_PRIO_IRQON; + childregs->pstate = PSR_MODE_EL1h | PSR_IL_BIT; p->thread.cpu_context.x19 = stack_start; p->thread.cpu_context.x20 = stk_sz; -- cgit v1.2.3